[analyzer][NFC] Refactoring BugReporter.cpp P4.: If it can be const, make it const

[analyzer][NFC] Refactoring BugReporter.cpp P4.: If it can be const, make it const

When I'm new to a file/codebase, I personally find C++'s strong static type
system to be a great aid. BugReporter.cpp is still painful to read however:
function calls are made with mile long parameter lists, seemingly all of them
taken with a non-const reference/pointer. This patch fixes nothing but this:
make a few things const, and hammer it until it compiles.

Differential Revision: https://reviews.llvm.org/D65382

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@368735 91177308-0d34-0410-b5e6-96231b3b80d8